Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Adds networking-sfc package

    Until networking sfc is added by:
    https://review.openstack.org/#/c/468703/
    We need to include it in our build system.

    Change-Id: If0332d6fb9a342d46599ca4ca556b2c1c7f01272
    Signed-off-by: Tim Rozet <trozet@redhat.com>
    (cherry picked from commit a622216b83fd46d3ad1fd763a367cc26323a8864)

diff --git a/docs/submodules/apex b/docs/submodules/apex
index 36e9307..c9c7bad 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit 36e93077409a1ac320a971fb4db5e95e3c5c0353
+Subproject commit c9c7bad8f5fc1ac458db50dd1882e5075871a15c